Lactic acid bacteria overproducing exopolysaccharides

ABSTRACT

The invention concerns lactic acid bacteria overproducing exopolysaccharides following mutation in the gene coding for α-phosphoglucomutase. Said mutants are useful, in particular for preparing fermented products or for producing exopolysaccharides.

The invention relates to the regulation of exocellular heteropolysaccharide production by lactic acid bacteria.

In general, polysaccharides are used a great deal as additives in food, but also in cosmetics and pharmaceutical products, for example as thickeners and/or gelling agents, texture stabilizers, fat substitutes, etc. Among the polysaccharides used in this way, mention will in particular be made of those produced by microorganisms, in particular bacteria, such as dextrans, xanthans, gellans, pullulans, etc.

Many species of lactic acid bacteria, in particular of lactococci, such as Lactococcus lactis, of leuconostocs, such as Leuconostoc mesenteroide, of streptococci, such as Streptococcus thermophilus, and of lactobacilli, such as Lactobacillus casei, Lactobacillus sake, Lactobacillus rhamnosus, Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus helveticus, etc., produce polysaccharides.

These polysaccharides can be grouped into 2 categories: homopolysaccharides, such as dextrans, which result from the polymerization of a single sugar, and heteropolysaccharides, which have a complex structure, combining basic units consisting of two or more different sugars (commonly D-galactose, D-glucose and L-rhamnose).

The heteropolysaccharides of lactic acid bacteria are conventionally designated under the general term EPS (for exopolysaccharides), which will also be used hereinafter. They play a major role in the development of the texture, of the mouthfeel and of the rheology of fermented dairy products. In addition, it has been observed that some of them have biological activities by which they might exert diverse effects beneficial to the health [for review, cf. DE VUYST and DEGEEST, FEMS Microbiology Reviews, 23, 153-177, (1999)].

However, the amount of heteropolysaccharides produced by lactic acid bacteria is generally low (of the order of 10 to 200 mg per liter of fermented product). To improve the texture, manufacturers of fermented products add other texturing agents, such as stabilizers (modified starches, carrageenan, guar, pectin, gelatin, etc.). However, these additions are not always authorized (for example in natural yogurt), and generally affect the taste and aroma of the product. Optimized production of exocellular heteropolysaccharides (EPS) in the product is therefore preferable.

Given the importance of EPS in the agrofoods industries, mentioned above, many studies have related to methods for increasing the production thereof by optimizing the biotechnological processes by acting on the temperature, the pH and the composition of the medium. These approaches are, however, sometimes difficult to apply in the context of certain agrofoods processes, such as the production of fermented dairy products for which the medium and the fermentation conditions are specific to each type of product. These production constraints therefore limit the use of conventional methods for optimizing the processes in order to improve EPS production. An alternative to these methods would be to use strains suitable for these processes, i.e. strains capable of producing EPS in greater quantity, and/or in which the EPS production may be controlled under the conditions for producing fermented products.

One of the main limitations of EPS production by lactic acid bacteria may come from competition between the biosynthesis of these EPS and other metabolic pathways for the use of available sugars. In fact, although homopolysaccharides are mainly produced by specific extracellular enzymes from substrates present in the medium, the synthesis of heteropolysaccharides takes place at least in part inside the cell. In particular, the formation of heteropolysaccharide precursors (nucleotide sugars), consisting of sugars activated by reaction with triphosphate nucleotides, might involve intracytoplasmic enzymes which also contribute to other metabolic pathways, and in particular to glycolysis. Now, due to the fermentative metabolism of lactic acid bacteria, the glycolysis reactions are more active than those relating to EPS synthesis.

The inventors set themselves the aim of obtaining lactic acid bacteria mutants in which it is possible to control and in particular increase the capacity to use sugars available in the medium, and in particular galactose, to produce EPS.

With this aim, they have investigated the genes involved in the synthesis of nucleotide sugars, the precursors of EPS, and/or the genes involved at the crossroads between the pathways of glycolysis and of EPS biosynthesis. Among the latter, they were more particularly interested in the genes for phosphoglucomutase (PGM), which is involved in the transformation of metabolic derivatives of galactose into glycolysis intermediates, and for glucose-1-phosphate uridyltransferase, which catalyzes the formation of nucleotide sugars, the precursors of EPS.

DE VUYST and DEGEEST (abovementioned publication) put forward the hypothesis that phosphoglucomutase might play an important linking role between glycolysis and EPS biosynthesis, and that diverting part of the carbon flow to this enzyme might make it possible to increase EPS production. However, they also underline that it remains to be seen whether this can effectively be done.

It has recently been reported [HARDY et al., J Bacteriol., 1854-1863, (2000)] that inactivation in Streptococcus pneumoniae of a gene [GENBANK AF165218] encoding a phosphoglucomutase leads to a dramatic decrease in the production of EPS forming the bacterial capsule, and is also highly damaging to cell viability, although this bacterium possesses another pgm gene.

The existence of an α-PGM and a β-PGM has been reported in L. lactis; however, only the gene corresponding to β-PGM has been isolated [QIAN et al., Microbiology, 143, 855-865, (1997)]. No gene capable of encoding an α-PGM of a lactic acid bacterium has been genetically characterized to date.

The inventors have now succeeded in cloning and characterizing a pgm gene encoding an α-PGM of Streptococcus thermophilus.

This gene is represented in the attached sequence listing under the number SEQ ID NO: 1, and the corresponding polypeptide is represented under the number SEQ ID NO: 2.

A subject of the present invention is a nucleic acid sequence encoding an α-PGM the amino acid sequence of which exhibits at least 70% identity or at least 85% similarity, preferably 80% identity or at least 90% similarity, advantageously at least 90% identity or at least 95% similarity, and most preferably at least 95% identity or at least 99% similarity, with the α-PGM represented by the sequence SEQ ID NO: 2, and also any fragment of more than 20 bp of said sequence.

The percentage identity of a sequence with a reference sequence is defined herein as the percentage of residues of this sequence which are identical to those of the reference sequence when the 2 sequences are aligned so that the positions of the residues correspond to a maximum. A polypeptide the amino acid sequence of which exhibits at least X % identity with a reference sequence can thus comprise up to 100-X modifications per 100 amino acids of the reference sequence. These modifications include the deletion, substitution or insertion of amino acid residues, which may or may not be consecutive.

The percentage similarity of a sequence with a reference sequence is defined herein as the percentage of residues of this sequence which are identical with those of the reference sequence, or which differ therefrom only by one conservative substitution, when the 2 sequences are aligned so that the positions of the residues correspond to a maximum. The term “conservative substitution” is intended to mean the substitution of an amino acid residue with another residue having similar physicochemical characteristics (size, charge or polarity) which do not change the functional properties of the protein. A polypeptide the amino acid sequence of which exhibits at least X % similarity with a reference sequence can thus comprise up to 100-X nonconservative modifications per 100 amino acids of the reference sequence. These modifications include the deletion, nonconservative substitution, or insertion of amino acid residues, which may or may not be consecutive.

The polypeptides thus exhibiting the greatest percentage identities or similarities with the sequence SEQ ID NO: 2, identified by searching on the “GENBANK nr” database using the BLASTp program [ALTSCHUL et al., Nucleic Acids Res., 25, 3389-3402, (1997)], with the default parameters, are as follows:

-   -   the ybbT protein of Bacillus subtilis: 57% identity and 69%         similarity;     -   the femD protein of Staphylococcus aureus: 53% identity and 69%         similarity;     -   the hypothetical phosphoglucomutase of Streptomyces coelicolor:         42% identity and 55% similarity;     -   the mrsA homologue of Pseudomonas syringae: 41% identity and 54%         similarity;     -   the mrsA homologue of Mycobacterium leprae: 41% identity and 54%         similarity.

No significant homology with the pgm gene of L. lactis described by QIAN et al. (publication cited above) was observed. The percentage identity with the pgm gene of S. pneumoniae described by HARDY et al. (publication cited above) is less than 31%.

The inventors have performed site-directed mutagenesis of the pgm gene of S. thermophilus, and have noted that, surprisingly, total or partial inactivation of this gene leads to an increase in EPS production.

A subject of the present invention is also a lactic acid bacteria mutant overproducing EPS, in which the pgm gene of alpha-phosphoglucomutase is totally or partially inactivated.

Said lactic acid bacterium will preferably be a mesophilic or thermophilic bacterium, chosen from streptococci and lactobacilli. By way of example, it may be Lactococcus lactis, Streptococcus thermophilus, Leuconostoc mesenteroide, Lactobacillus casei, Lactobacillus delbrueckii, Lactobacillus sake, etc.

Inactivation of the pgm gene may be obtained by making one or more mutations in the sequence encoding α-PGM, and/or in sequences controlling its expression.

Use may in particular be made of site-directed mutagenesis techniques, which are in themselves known to those skilled in the art and which make it possible to introduce a defined mutation into a gene, at the desired position.

It is thus possible, for example, to inactivate the pgm gene by introducing into the coding sequence, or the regulatory sequences, an exogenous sequence, for example a transposon.

Advantageously, it is also possible to replace the wild-type sequence of the pgm, by homologous recombination, with the mutated sequence. In this case, one or more modifications by insertion, deletion or substitution of one or more nucleotides, which may or may not be consecutive, are made in a sequence identical to that of the region of the gene intended to be mutated.

According to a preferred embodiment of the present invention, the mutated sequence is inserted into a vector which allows integration by recombination (simple crossing-over) between the bacterial DNA fragment cloned into the vector and the homologous region of the bacterial genome. Excision of the vector sequences can then occur following a second recombination event (double crossing-over), which results in substitution of the wild-type chromosomal form by the modified form.

Vectors which allow the integration of an exogenous sequence into the chromosome of a lactic acid bacterium are known in themselves, and are available for most species of lactic acid bacteria; they may, for example, be nonreplicating vectors, unstable replicative vectors or vectors which replicate conditionally, vectors carrying insertion sequences, etc.

In certain cases, it is also possible to transform the bacterium directly with the DNA carrying the mutated sequence intended to be inserted.

Mutants of the pgm gene in accordance with the invention can also be obtained by random mutagenesis (for example by chemical mutagenesis or radiation); they may also be natural mutants selected from cultures of lactic acid bacteria by screening on the basis of their phenotypic properties.

In fact, the mutants in which the α-PGM is partially or totally inactive have normal growth on lactose, and very slow growth on glucose or galactose alone. Such mutants, whether they are natural mutants or are derived from mutagenesis, can therefore be selected directly on the basis of this property.

The presence of mutations in the pgm gene, and the nature of these mutations, can be easily verified with conventional molecular biology techniques, using nucleic acid primers or probes or strains derived from the sequence SEQ ID NO: 1.

If desired, it is also possible to increase or decrease EPS production at will, as a function, for example, of the culturing conditions, by placing the pgm gene under the transcriptional control of an inducible promoter.

The inventors have also cloned and characterized the galU gene which encodes S. thermophilus glucose-1-phosphate uridyltransferase. This gene is represented in the attached sequence listing under the number SEQ ID NO: 3. The GalU protein encoded by this gene is represented under the number SEQ ID NO: 4.

A subject of the present invention is also a nucleic acid sequence encoding a glucose-1-phosphate uridyltransferase the amino acid sequence of which exhibits at least 90% identity or at least 95% similarity, and advantageously at least 95% identity or at least 99% similarity, with the glucose-1-phosphate uridyltransferase represented by the sequence SEQ ID NO: 4.

The polypeptides thus exhibiting the greatest percentage identities or similarities with the sequence SEQ ID NO: 4, identified by searching on the “GENBANK nr” database using the BLASTp program [ALTSCHUL et al., Nucleic Acids Res., 25, 3389-3402, (1997)], are as follows:

-   -   the glucose-1-phosphate uridyltransferase of Streptococcus         mutans: 87% identity and 93% similarity;     -   the GalU protein of Streptococcus pneumoniae: 87% identity and         93% similarity;     -   the UDP-glucose pyrophosphorylase of Streptococcus pyogenes: 84%         identity and 90% similarity;     -   the Cap3C protein of Streptococcus pneumoniae: 76% identity and         88% similarity;     -   the UDP-glucose pyrophosphorylase of Bacillus subtilis: 55%         identity and 74% similarity.

Overexpression of the galU gene makes it possible to increase EPS synthesis by increasing the amount of precursors thereof.

A subject of the present invention is therefore also a lactic acid bacterium mutant overproducing EPS, in which the galU gene is overexpressed. Such a mutant may in particular be obtained by introducing one or more copies of this gene into a lactic acid bacterium, and/or replacing its promoter with a strong promoter. Preferably, said mutant is obtained from a lactic acid bacterium also having a partially or totally inactive α-PGM.

Mutant strains in accordance with the invention may in particular be obtained from S. thermophilus, and in particular from S. thermophilus strains selected for their ability to grow on galactose. In fact, in this bacterium, another factor which is limiting for EPS synthesis may come from the fact that a large number of strains do not effectively use galactose, whether for growth or EPS synthesis.

The inventors have noted that gal+ strains of S. thermophilus, selected for their ability to grow on galactose, can synthesize a larger amount of EPS. Using these clones capable of using galactose for their growth, it is possible to carry out a second selection of the strains producing colonies which appear to be larger in volume, which reflects greater synthesis of EPS.

The use of these gal+ strains of S. thermophilus, as starting material for the production of mutants in accordance with the invention, therefore further improves the ability thereof to produce EPS.

Lactic acid bacterial strains in accordance with the invention can advantageously be used for producing fermented products, in particular food products, in which it is desired to control, and in particular increase, the EPS content, and also for producing EPS.

The present invention will be clearly understood by virtue of the further description which follows, which refers to examples of obtaining and using lactic acid bacterial strains in accordance with the invention.

EXAMPLE 1 Selecting S. Thermophilus Strains Using Galactose more Effectively for EPS Synthesis

A) Strains capable of effectively using galactose were selected using cultures of industrial leavens of S. thermophilus from the RHODIA and DANONE collections: RD488 (JIM7446) (RHODIA collection) and ext 1.5 (JIM7459) (DANONE collection).

For this, a 12-hour culture in M17 medium [TERZAGHI and SANDINE, Appl. Microbiol., 29, 807-813, (1975)] containing 1% of lactose is deposited onto M17 agar plates containing 1% of galactose, and is incubated at 42° C. for 2 days. The colonies thus obtained are streaked onto an M17 agar plate containing 1% of galactose, and are incubated at 42° C. overnight. The colonies thus growing on M17 galactose are recovered and grown in liquid M17 containing 1% galactose at 42° C. for 6 hours before being placed in a collection. Other media, such as the chemically defined medium described by SISSLER et al. [Proc. Natl. Acad. Sci. USA, 96, No. 16, 8985-8990, (1999)], in which the sugar source can be controlled, can also be used for this selection.

In order to verify that the strains obtained are indeed capable of more effectively using galactose, the growth of these strains and of the parents thereof are compared.

The growths are carried out as follows: overnight precultures in 10 ml of BELLIKER medium (DIFCO Laboratories) to which 10 g of beef extract/1 (DFICO) are added, and containing 1% of galactose, are diluted 100-fold and grown in 2 ml of M17 medium containing 2% of sugar (sucrose, lactose, glucose or galactose) for 4 hours at 42° C. They are then inoculated at 2% into 200 μl of M17 medium containing 1% of sugar. The growth of the strains is then followed using the BIOSCREEN system (LABSYSTEMS).

FIG. 1 represents the growth, on various sugars, of the industrial strain JIM7459 (1A) and of a gal+ mutant selected from this strain (1B).

B) It is possible to carry out a further selection from clones already capable of using galactose for growth. Clones producing colonies which appear to have a greater volume are then sought on the plates. These strains use galactose even more effectively.

FIG. 2 represents the growth, on various sugars, of a gal+ mutant (2A) selected as described in A above, from a culture of an industrial strain (JIM7446, DANONE collection), and of a gal++ mutant (2B) selected from a culture of this gal+ mutant on the basis of colony size.

After growth of these various strains in chemically defined medium, or in M17 medium, the bacterial pellet of the gal+ mutants is more filamentous than that of the wild-type strains, which may reflect a difference in EPS production between the mutants obtained and their parental strain. The EPS production of these strains was determined from a culture in 80 ml of chemically defined medium [SISSLER et al., Proc. Natl. Acad. Sci. USA, 96, No. 16, 8985-8990, (1999)].

After growth, the cultures are centrifuged at 16 000 g for 10 min at 4° C. The cell pellet is removed and the supernatant is precipitated (2 volumes of 100% ethanol per volume of supernatant) for 24 hours at 4° C. After precipitation, a centrifugation is performed at 16 000 g for 15 minutes at 4° C. The supernatant is removed and the pellet is resuspended in 60 ml of water. Dialysis is performed against water for 4 days, exchanging the water 3 to 4 times per day. The EPS are assayed using the phenol/sulfuric acid method [DUBOIS et al., Analytical Chemistry, 28, 350-356, (1956)].

The results obtained in the case of the 7446 Gal+ strain, and of a mutant of this strain capable of using galactose more effectively, are shown in table I below.

TABLE I STRAINS EPS (mg/l) 7446 Gal+ 4 7446 Gal++ 8

After growth in the presence of galactose, the strains using galactose more effectively produce increased amounts of EPS.

EXAMPLE 2 Cloning the PGM Gene of Streptococcus Thermophilus and Constructing Mutants in which this Gene is Inactive

Cloning the Gene:

The pgm gene of Streptococcus thermophilus can be obtained by reverse PCR (OCHMAN et al., Biotechnology (NY), 8, No. 8, 759-760, (1990)] from the chromosomal DNA of S. thermophilus. The chromosomal DNA of S. thermophilus is digested with restriction enzymes (BamHi, EcoRI, HindIII, NcoI, PsI, XhI) and the cleavage products are then circularized then amplified by PCR using the primers complementary to the opposite strand: OST15, OST16, OSY23 to OST26 (table II).

TABLE II Primer Sequence OST15 ACATAACCACCAAAACGACCTAA OST16 GGAAGCTGAGATGGCTGGTA OST23 TCTTCAAGAACTGCACGGTCA OST24 CAAAATTCTTCGTCTTTACCG OST25 GCTAGCCTTATCGCCAGTCAA OST26 TTGAGTAAATCAGTTCCAGTT

The bands obtained are extracted from the gel and sequenced. The sequence of the cloned fragment, comprising the pgm gene and its flanking regions, is represented in the attached sequence listing under the number SEQ ID NO: 1.

The length of the open reading frame of the pgm gene of Streptococcus thermophilus is 1350 bp.

Constructing Mutants of the pgm Gene

In order to decrease the activity of the pgm gene, the inventors adopted a strategy of inactivation by insertion of a vector into the gene by homologous recombination. The general strategy of inactivation is represented in FIG. 3. Initially, thermosensitive replication plasmids containing fragments internal to the pgm gene were constructed using the thermosensitive replication vector pG+host [BISWAS et al., J. Bacteriol., 175, 11, 3628-3635, (1993); PCT application WO/181164]. Two PCR fragments produced with the oligonucleotides OST29+OST31 and OST28+OST31, and representing, respectively, 650 and 835 bp of the pgm gene, were cloned into pG+host, generating the plasmids pST28 and pST29.

TABLE III Primer Sequence OST28 TAAGGGCCCTAAATATTTTGGAA CAGA OST29 CCGGGGCCCACTTCTCTCAGTAG GTAT OST31 AATATCGATATTTTCATCAACGGC AAT

The 650 bp fragment carried by pST28 is central to the gene, it is therefore expected that the insertion thereof should produce total inactivation of the gene. The 835 bp fragment carried by pST29 contains the portion located in the 5′ position of the gene with the translation initiation codon, but without its ribosome binding site. It is expected that the insertion of pST29 into the chromosome will cause a very large decrease in translation of pgm, allowing only a very low base expression.

The plasmids pG+host, pST28 and pST29 were introduced by transformation into the S. thermophilus strains JIM7446 and JIM7459 at 30° C., and then integrated by simple crossing-over, as shown diagrammatically in FIG. 3. The integration by simple crossing-over was carried out according to the protocol described by BISWAS et al. [publication cited above, (1993)] with the following modification:

The Streptococcus thermophilus strains containing the plasmid pG+host or its derivatives are grown overnight at 30° C. in the presence of erythromycin, and then diluted 50-fold in the same medium. They are then transferred to 42° C. for 6 hours. The samples are then diluted and plated out, firstly, at 42° C. on M17 plates containing erythromycin in order to detect the integration events and, secondly, at 30° C. on M17 plates without antibiotic in order to detect the total number of viable cells. The frequency of integration per cell is calculated from the ratio of these counts. The frequencies of integration of these plasmids are, respectively, 3.5×10⁻³, 3×10⁻³ and 10⁻² for pG+host, pST28 and pST29. Clones derived from each of these integrations were isolated to give the strains STJ3, STJ1 and STJ2, respectively.

The STJ1 (pST28), STJ2 (pST29) and STJ3 (pG+host) strains were grown on plates of chemically defined medium containing glucose and galactose or on plates of the same media containing either glucose alone or galactose alone. The results obtained are given in table IV.

TABLE 4 Growth on CDM Growth on CDM Growth on CDM Plasmid strains Glu Gal Ery Glu Ery Gal Ery STJ1 pST28 Normal No growth No growth STJ2 pST29 Normal No growth No growth STJ3 pG + host Normal Normal Normal

The STJ1 and STJ2 clones do not therefore grow on glucose or galactose alone, but grow normally in lactose or on a glucose and galactose mixture. This shows that glucose and galactose metabolism has indeed been uncoupled in this strain and that the gene whose activity has been affected is indeed pgm.

These results show that the inactivated gene clearly encodes an enzyme which connects the EPS pathway and glycolysis. It therefore probably encodes α-PGM, the sequence of which had not yet been characterized experimentally in lactic acid bacteria.

Assaying the EPS Produced by the Streptococcus thermophilus Strain STJ2 Carrying a Mutation in the pgm Gene

The EPS are assayed as described in example 1 above.

The EPS production in the strain affected in terms of expression of the pgm gene is 20 mg/l. This production, compared to the 8 mg/l obtained for the parental strain 7446 Gal++, therefore corresponds to a two and a half-fold increase.

EXAMPLE 3 EPS Production by pgm Gene Mutants Derived From Industrial Strains

The plasmid pST29 described in example 2 above was used to transform the industrial strains (DANONE collection) Ext 1.1 (JIM7455) and Ext 1.10 (JIM7464). These strains produce EPS with different compositions (1-glu:2-gal: 1-galNac for Ext 1.1 and 3-gal: 1-rha for Ext 1.10), and which are also different from that produced by the JIM7446 strain (3-glu:4-gal).

After electroporation, the cells are plated out at 30° C. for 12 h and the plates are then placed at 42° C. in order to force integration of the plasmid pST29 into the pgm gene by homologous recombination between the pgm sequences present on the plasmid and those present on the chromosome. Colonies of mutated bacteria capable of growing at 42° C. in the presence of erythromycin were obtained. These bacteria are also capable of growing on CDM (chemically defined medium), in the presence of glucose and galactose, but not in the presence of glucose alone or galactose alone.

The transformed or untransformed strains were cultured in 10 ml of CDM containing 2% lactose, 1.5% casitone and 10 mM urea, in the presence of erythromycin (5 μg/ml) for the strains transformed with the plasmid pST29. The EPS produced by the various strains were assayed after 67 h 30 min of culturing at 42° C. The results obtained are given in table V below.

TABLE V Strain OD₆₀₀ pH mg EPS/l mg EPS/l/OD 7455 (Ext 1.1) 1.566 4.6 12.5 8 7455::pST29 0.98 4.6 14.68 16 7464 (Ext 1.10) 1.29 4.24 23.6 18.29 7464::pST29 0.99 4.2 44 44

It is noted that the strains in which the pgm gene is inactivated in accordance with the invention produce, at equal biomass, 2 to 2.5 times more EPS than the strains of origin.

EXAMPLE 4 Constructing a Mutant of the pgm Gene by Double Crossing-Over

A vector named pSTJ6, containing a pgm gene inactivated by internal mutation, was constructed by reverse PCR amplification using the plasmid pST29 as matrix and the following primers, derived from the sequence of the pgm gene with point mutations, inducing in particular the creation of an NsiI site:

GAC A TGC AT CGCTTGTCGATTAGCCAGAAGGTC GCG A TGC AT GTCATTAAGCTGATGGGCGTCGAAG

The mutations introduced into the primers are underlined: the NsiI site created is indicated in italics.

The PCR product is cleaved with NsiI and religated on itself to give the vector pSTJ6.

This vector is introduced by transformation into the S. thermophilus strain JIM7446.

The colonies are plated out at 30° C. for 12 h, and then grown at 42° C. for 24 h in order to force the integration of the vector into the chromosome via a first crossing-over by recombination between the pgm sequences present on the plasmid and those present on the chromosome. The colonies thus obtained, erythromycin resistant and capable of growing at 42° C., are then grown at 30° C. in order to promote excision of the pG+host plasmid sequences via a second crossing-over by recombination between the pgm sequences present on either side of the pG+host sequences. pG+host is then removed from the cell by a step of growth at 42° C. in the absence of selection with erythromycin. The strain thus obtained no longer contains any sequences of the vector pG+host and, in particular, it does not contain a gene for resistance to antibiotics.

It differs from the S. thermophilus strain of origin, JIM7446, only by the presence of the mutations in a portion of the pgm gene. The initial sequence and the mutated sequence of this portion of the pgm gene are shown below; the mutations are underlined.

Initial sequence:

GCTGAAGGACTTGGAACGCTTGTCGATTATCCA Mutated sequence:

GCTTAATGACATGCATCGCTTGTCGATTAGCCA

The mutated strain has the phenotype characteristic of the pgm mutants, described in examples 2 and 3 above. It is capable of growing on CDM in the presence of glucose and galactose but not in the presence of glucose alone or galactose alone. The increase in EPS production by this strain, relative to the parental strain, is comparable to that observed in the case of the strains transformed with pST29, described in examples 2 and 3.

EXAMPLE 5 Constructing Mutants Overproducing galU

Cloning the galU gene encoding glucose-1-phosphate uridyltransferase

The entire galU gene can be obtained by reverse PCR [OCHMAN et al. (1990), publication cited above] from the chromosomal DNA of S. thermophilus, using primers complementary to the opposite strand (OST13, OST14, OST21 and OST22) (table VI).

TABLE VI Primer Sequence OST13 CCAAGATATCTTCAATACCAGAC OST14 GAAAACACAACGCGTCTTTGC OST21 GCACCAGCACCGACTGCGATA OST22 ACTGGGTAGTTAAACGGTAAT

The bands obtained are extracted from the gel and sequenced.

The sequence of the cloned fragment, comprising the galU gene, is represented in the attached sequence listing under the number SEQ ID NO: 3.

The size of the galU gene is 914 bp.

Overexpression of GalU

The galU gene (with its terminator and its ribosome binding site) was amplified with the oligonucleotides OST44 and OST45 (table VI) and cloned downstream of the p45 promoter [SIBAKOV et al., Appl. Environ. Microbiol., 57, 341-348, (1991)] of L. lactis in the ApaI site.

TABLE VI Primer Sequence OST44 AATGGGCCCAAAAATAAAAAATCTAAG GAG OST45 AAGCTGCAGACTTATCTTTAAATTAAAT GA

A SmaI fragment of the resulting plasmid, carrying P45 followed by galU, was purified and inserted into the SmaI site of the vector pGKV259 which replicates in S. thermophilus. The plasmid pGKV259 [VAN DER VOSSEN et al., Appl. Environ. Microbiol., 53, 2452-2457, (1987)] expressing the galU gene under the control of the p45 promoter is called pSTJ4 (FIG. 4). This plasmid is introduced by transformation into the S. thermophilus strain JIM7446 or JIM7459 or into the strain containing a mutation in the pgm gene (STJ2). 

1. A lactic acid bacterium having a mutation in the gene encoding a protein having an amino acid sequence with at least 80% identity to SEQ ID NO: 2, wherein said mutation totally or partially inactivates said gene and enhances the production of exopolysaccharides by said bacterium.
 2. The mutant as claimed in claim 1, wherein a gene encoding a protein having an amino acid sequence with at least 95% identity to SEQ ID NO: 4 is overexpressed, and wherein said overexpression further enhances the production of exopolysaccharides by said bacterium.
 3. The mutant as claimed in claim 1, characterized in that said lactic acid bacterium is Streptococcus thermophilus.
 4. The mutant as claimed in claim 1, characterized in that it is obtained from a Streptococcus thermophilus strain capable of growing on galactose.
 5. The mutant as claimed in claim 1, wherein said gene encodes a protein having an amino acid sequence with at least 90% to SEQ ID NO:
 2. 